Porous materials have been extensively studied and are used for a wide range of applications, including as ion exchangers and drying agents.1,2 Porous materials constructed using organic templates do not usually have accessible porosity until the template is removed. For example, mesoporous silica (MCM-41), first reported in 1992, is prepared by templating the condensation of silica around a lyotropic liquid crystalline phase followed by calcination of the template.3-5 Besides calcination, other methods have been used to remove neutral or charged organic or inorganic templates from inside of a porous silica-based material, including acid-extraction and solvent-extraction.6-8 In particular, methods such as solvent extraction and acid-extraction are used to prepare mesoporous organosilicas since the organic group in the wall of the material cannot usually withstand the high temperature conditions of the calcination.9-11 Mesoporous organosilica materials can exhibit unique properties compared to mesoporous silica such as enhanced hydrothermal stability, chemical stability, and mechanical properties.12,13 This class of materials is therefore of great interest for a variety of potential applications.
Cellulose has been used in various forms to construct cellulose-silica composites.14-16 Where it has been removed to afford a porous structure, the cellulose has been calcined under air or oxygen. We recently reported a new type of silica-cellulose composite material where nanocrystalline cellulose is organized in a chiral nematic assembly inside of the composite.17 After calcination, the nanocrystalline cellulose is decomposed, leaving a porous, chiral nematic silica material. One drawback of this method is that the pores in the material are smaller than the diameter of the individual NCC crystallites owing to condensation and collapse of the pores during calcination. Another significant drawback is that it does not allow for the incorporation of organic groups or other temperature-sensitive groups into the silica walls as they generally will thermally decompose at the temperatures required to degrade cellulose.
The decomposition of cellulose by a strong acid (e.g., HCl, H2SO4) in water, ionic liquids and other solvents has been extensively studied.18-20 Much of this research has been aimed at converting cellulose to glucose, which may then be converted to ethanol for use as a biofuel. Under these circumstances the conditions must be selected very carefully to avoid the formation of other byproducts of cellulose decomposition. Acid-catalyzed hydrolysis of cellulose has not been applied to the removal of cellulose from silica-cellulose or organosilica-cellulose composite materials, where it can generate properties distinct from those where the cellulose was calcined. In this case, the specific degradation products of cellulose are relatively unimportant so long as the cellulosic material is effectively removed from the silica or organosilica network without structural damage to the network.